Thallium germanate, tellurite, and antimonite glasses

ABSTRACT

This invention relates to thallium tellurite glasses possessing a high Verdet constant and high optical nonlinearity, as well as good visible and infrared transmission, making them suitable materials for the fabrication of active optical devices. The glasses consist essentially, expressed in terms of cation percent, of 5-74.5% TeO 2 , 0.5-20% SiO 2 , 4-50% TlO 0 .5, and 0-71% PbO, wherein PbO+T10 0 .5≧25 %, TeO 2  +SiO 2  ≧25%, and TeO 2  +SiO 2  +TlO 0 .5 +PbO constitutes at least 65% of the total composition.

This application is a Continuation-In-Part application of U.S. Ser. No.07/813,536, filed Dec. 26, 1991, now abandoned, which, in turn, was adivision of U.S. Ser. No. 07/618,939, filed Nov. 28, 1990, now U.S. Pat.No. 5,093,288.

RELATED APPLICATION

U.S. Ser. No. 618,938, filed concurrently herewith by N. F. Borrelli etal. under the title GALLIOBISMUTHATE GLASSES, now U.S. Pat. No.5,093,287, is directed to the production of glasses with superiorinfrared transmitting ability and high optical nonlinearity, andconsisting essentially, in weight percent, of 42-48% PbO, 33-44% Bi₂ O₃,10-15% Ga₂ O₃, and up to 15% total of at least one member selected fromthe group consisting of up to 5% total SiO₂ and/or GeO₂ and up to 15%Tl₂ O.

BACKGROUND OF THE INVENTION

Active optical devices, which are currently being studied for use infuture telecommunications and computational systems, require materialsthat are characterized by large optical nonlinearity. In such materialsa light beam will cause a significant change in the optical propertiesof the medium, altering the propagation of other light beams(cross-modulation) or of itself (self-modulation). Many optical devicesfurther require that the materials have a fast response time and lowoptical loss. Glasses containing high concentrations of heavy metaloxides (HMO) have been found to display a high degree of opticalnon-linearity which, due to its electronic nature, is furthercharacterized by a rapid response time. For example, measurements onrepresentative HMO glasses have shown a non-linear refractive index some50× that of vitreous silica. These features, coupled with excellenttransmission in both the visible and infrared portions of theelectromagnetic spectrum, render these materials ideal for a wide rangeof optical applications, including those where light induced phasechanges, beam steering, optical phase conjugation, and bistability ofresonator structures are desired.

Optical devices utilized in these applications require that the opticalproperties of the active material change in response to an appliedoptical field, the magnitude of this effect being characterized by asingle term called the third order susceptibility tensor, X.sup.(3). Thelatter can be thought of as a coefficient in a power series expansion ofthe relationship between the applied electric field, E, and thepolarization, P, written schematically as

    P=X.sup.(1) E+X.sup.(2) EE+X.sup.(3) EEE+. . . .

where X.sup.(1) is the linear susceptibility tensor, X.sup.(2) is thesecond order susceptibility tensor, and so on. X.sup.(1), the linearsusceptibility, is related to the linear refractive index, n_(o), byX.sup.(1) =(n_(o) ² -1)/4π.

When an optical field is applied to a material, such as a glass, theoptical properties of the material are affected. The magnitude of thiseffect can be measured, and is characterized in the art by X.sup.(3).Illustrative of this phenomenon is the example where a high-intensitylight beam is passed through a nonlinear medium, consequently inducing achange in the index of refraction. For a light beam of intensity I, theindex of refraction can be described by the equation

    n=n.sub.o +γI

where n is the index of refraction, n_(o) is the linear (low intensity)index, and γ is the nonlinear refractive index of refraction. Thenonlinear index, in turn, is related to X.sup.(3) by the equation

    γ=[(480π.sup.2)/(n.sub.o c.sup.2)]X.sup.(3).sub.1111

where X.sup.(3) ₁₁₁₁ is the first diagonal term of the third ordersusceptibility tensor and c is the velocity of light. In a nonlineardevice this change in refractive index is achieved when one light beamis used to manipulate the behavior of another.

One technique that may be used in measuring X.sup.(3) of various oxideglasses, called degenerate four wave mixing (DFWM), has shown that themaximum values of X.sup.(3) are attained in those glasses having thehighest concentration of the heavy metals thallium, lead, and bismuth.The ions of these metals, in their normal oxidation state in HMOglasses, i.e., T1⁺, Pb⁺⁺, and Bi⁺³, have an electronic configurationwhich is essentially that of an inert gas plus two 6s electrons.Strictly speaking, the electronic configuration of these ions is[Xe](4f¹⁴)(5d¹⁰)(6s²), but, as the filled 4f and 5d shells as well asthe xenon core comprise a spherically symmetric density, theconfiguration can be abbreviated as 6s². These two 6s electrons comprisea stereochemically active lone electron pair in that they occupy adirected orbital, but this orbital is essentially non-bonding incharacter. The presence of this lone electron pair is responsible forthe highly distorted geometry assumed by Tl-O, Pb-O, and, to a lesserextent, Bi-O coordination polyhedra, and is postulated to be the sourceof the high values of X.sup.(3) characteristic of HMO glasses.

DFWM measurements on a number of HMO glasses suggest that, of the threeheavy metal ions, T1⁺ has the largest overall contribution on anatom-for-atom basis to the net X.sup.(3) of an HMO glass. Thus, oxideglasses with high concentrations of thallium are here identified asbeing especially well suited for the fabrication of nonlinear opticaldevices.

The enhanced nonlinearity of the HMO glasses described herein andwaveguide structures that may be synthesized from these glasses makethem useful in a number of device configurations, some of which can onlybe implemented in waveguide form (either fiber or planar waveguides),and some of which can also be implemented in bulk-optics form. Suchdevices include, but are not limited to: nonlinear mode couplingdevices, nonlinear interference devices, and optical amplifiers, andareas where optical phase conjugation is desirable.

Nonlinear mode coupling devices operate through changing the coupling oftwo (or more) modes of a waveguide structure as a result of the thirdorder susceptibility. They include various multi-core couplers andsingle-core devices where two or more modes of the waveguide structure(such as modes with different polarizations or spatial distributions)have their coupling altered through nonlinear interaction.

In nonlinear interference devices the relative phases of two or morelight beams (or even various reflections of a single light beam) arechanged by utilizing variations in the optical path length resultingfrom the third order susceptibility. Such differences are brought aboutby the change of index of refraction due to the nonlinearity.Representative of this group of nonlinear interference devices is a bulkor guided wave Mach-Zehnder interferometer, although Sagnacinterferometers, Michelson-type interferometers, distributed feedbackgrating devices, and Fabry-Perot resonators may also be included.

When these HMO glasses are used in synthesizing optical amplifiers, thegain coefficients for stimulated Raman and Brillouin amplification arealso enhanced. This gain can be used to amplify a signal beam using apump beam in a guided-wave geometry.

In areas where optical phase conjugation is desired, four-wave mixinginteractions (bulk or guided wave) are utilized, wherein three inputoptical waves interact via the third order susceptibility to form afourth wave, called a phase conjugate wave, which has unique properties.These properties can be exploited for such uses as aberrationcorrections, optical memory, beam steering, generation of newwavelengths, and neural networks.

In summary, the primary objective of the present invention was to designnew glasses containing large concentrations of thallium, which glasseswill exhibit a high degree of optical nonlinearity as well as superiorvisible and infrared transmission, those properties rendering theglasses particularly suitable for use in the aforementioned generalareas of application.

SUMMARY OF THE INVENTION

The above goals can be achieved within narrow composition intervals ofthe following system:

TeO₂ -Tl₂ O-SiO₂ with, optionally, PbO

More specifically, the glasses of the instant invention are selectedfrom the following group and consist essentially, expressed in terms ofcation percent on the oxide basis, of:

5-74.5% TeO₂, 0.5-20% SiO₂, 4-50% TlO₀.5, and 0-71% PbO, wherein TlO₀.5+PbO≧25%, TeO₂ +SiO₂ ≧25%, and TeO₂ +SiO₂ +TlO₀.5 +PbO constituting atleast 65% of the total composition. Desirably, up to 35% total of atleast one member of the group in the indicated proportion of up to 35%WO₃ and up to 25% ZnO may be included.

Tl₂ O-Bi₂ O₃ -GeO₂ -Ga₂ O₃ glasses are especially well suited for thefabrication of nonlinear optical devices due to their highconcentrations of thallium. Thus, the stabilizing effect, i.e., theresistance of the glass to devitrification, imparted by Ga₂ O₃ and Bi₂O₃ permits thallium concentrations as high as 60 cation percent to beemployed without compromising glass stability, and allows thepreparation of such thallium-rich glasses with up to 85% total heavymetal content, i.e., TlO₀.5 +BiO₁.5. Such glasses would be expected toexhibit large nonlinear effects, and those effects have been confirmedthrough DFWM measurements.

Thallium tellurite glasses are also of great interest from thestandpoint of nonlinear optical applications. Although the maximumpermissible thallium concentrations in these glasses are less than 60%and thus lower than those attainable in the previous system, thenetwork-forming component tellurium, unlike germanium, is known tocontribute significantly to X.sup.(3). This contribution may be due tothe fact that the Te⁴⁺ ion, like the Tl⁺, Pb⁺⁺, and Bi³⁺ ions, also hasa stereochemically active lone electron pair, in this case 5s², which issimilarly expected to contribute to an enhanced value of X.sup.(3) intellurite glasses. In addition, these glasses exhibit excellent infraredtransmission, high Verdet constants, high electrooptic coefficients, aswell as good chemical durability relative to other thallium-bearingglasses such as thallium silicates, borates, and/or germanates. Thethallium tellurite glasses are further characterized by excellenttransmission in the visible portion of the electromagnetic spectrum.SiO₂ is included in the tellurite glass compositions to assure glassstability and to improve their forming properties. The glasses exhibitrelatively broad working ranges, generally greater than 100° C., therebyrendering easier the forming of glass articles such as glass fibers.Although the advantageous effects flowing from the inclusion of Tl₂ Oand SiO₂ can be felt at very low levels thereof, a level of 4% Tl₂ O anda level of 0.5% SiO₂ have been deemed to comprise practical minima forthe two components.

Whereas it is not mathematically possible to convert composition rangesexpressed in terms of cation percent to exact composition rangesexpressed in terms of weight percent, the following values representapproximations of the base compositions of the inventive glasses interms of weight percent:

4.3-69.0% TeO₂, 0.2-7.9% SiO₂, 4.1-63.9% Tl₂ O, and 0-84.8% PbO.

PRIOR ART

A plethora of glass compositions are known to the art that have infraredtransmitting characteristics. Included in such optical glasscompositions are, for example, isotropic crystalline alkaline earthfluorides (U.S. Pat. No. 3,502,386), antimony sulfides (U.S. Pat. No.3,002,842), germanium-arsenic-selenides/tellurides (U.S. Pat. No.4,154,503), and strontium and gallium compounds (U.S. Pat. No.3,188,216), as well as methods for making infrared transmittinggermanate glasses (U.S. Pat. No. 3,531,305). None of the glasscompositions mentioned in these references coincides with those of theinstant invention.

Oxides of tellurium, tungsten, tantalum, thallium, bismuth, barium,lead, and titanium were employed in synthesizing the optical glasses ofU.S. Pat. No. 3,291,620 (Evstropjev), whereas at least two members fromthe group consisting of tungsten, molybdenum, bismuth, and arsenic andone member selected from the group of oxides and fluorides consisting ofmagnesium, calcium, strontium, barium, and lead, were utilized in U.S.Pat. No. 3,531,304 (Bromer). Neither reference mentions the use of Ga₂O₃.

Optical waveguide fibers have been constructed from some of theseinfrared transmitting optical glasses as illustrated in U.S. Pat. Nos.3,209,641 and 4,451,116, wherein fibers were constructed from arsenicand sulfur, and fibers were extruded from halides of thallium,respectively.

U.S. Pat. No. 3,723,141 (Dumbaugh, Jr.) is directed to the formation ofinfrared transmitting glasses consisting essentially, expressed in termsof weight percent on the oxide basis, of 10-85% Bi₂ O₃, 10-75% PbO, Bi₂O₃ +PbO>60%, 2-25% BaO, 1-10% ZnO, SiO₂ +B₂ O₃ +P₂ O₅ <1%, and up to 20%total of the following components in amounts not exceeding 10%individually: CaO, SrO, CdO, HgO, Tl₂ O₃, TiO₂, GeO₂, Sb₂ O₃, As₂ O₃,the transition metal oxides, and the alkali metal oxides. Ga₂ O₃ andTeO₂ are nowhere mentioned in the patent.

U.S. Pat. No. 3,837,867 (Dumbaugh, Jr.) is concerned with infraredtransmitting glasses consisting essentially, expressed in terms ofcation percent on the oxide basis, of 33-68% PbO, 2.5-27% CdO, 10-30%Fe₂ O₃, and 4-28% Tl₂ O. Those ranges correspond to the followingapproximate weight percentages: 40-80% PbO, 4-35% Tl₂ O, 2-22% CdO, and4-15% Fe₂ O₃. Ga₂ O₃ and TeO₂ are nowhere referred to in the patent.

In "Properties and Structure of the Binary Tellurite Glasses ContainingMono- and Di-valent Cations," Ceramic Society Journal, 86 (7) 1978, p.317-326, authored by N. Mochida, K. Nakata, and S. Shibusawa, the glassformation ranges of the binary tellurite systems MO_(1/2) -TeO₂ (where Mis Li, Na, K, Rb, Cs, Ag, or Tl) and MO-TeO₂ (where M is Be, Mg, Ca, Sr,Ba, Zn, Cd, or Pb) were investigated. In that work there is nodiscussion of ternary or quaternary glass systems, of the inclusion oftrivalent and quadrivalent ions such as Bi⁺³, Sb⁺³, and Si⁺⁴, or of thenonlinear optical properties of tellurite glasses and of the possibleuse of said glasses in the fabrication of active optical devices.

In "Glass Formation in the System GeO₂ -Bi₂ O₃ -Tl₂ O," Journal of theAmerican Ceramic Society, 65(1982), 197-203, authored by K. Nassau andD. L. Chadwick, the occurrence of glasses in the ternary oxide systemcontaining Ge, Bi, and Tl was studied. No reference was made to Ga₂ O₃so there was no recognition of its significant effect upon the stabilityof the ternary glasses. Furthermore, there was no discussion of glassesexpressly devised for applications in active optical devices.

The journal article immediately above was referred to in U.S. Pat. No.4,790,619 (Lines et al.), the object of the patent being to produce aRaman-active optical fiber of very high Raman cross section utilizingglass compositions described in that journal article among others. Thus,the patentees directed their invention to core glasses of a Raman-activeoptical fiber comprising a glass forming first major component selectedfrom the group consisting of GeO₂, SiO₂, AsO₁.5, and combinationsthereof, and a heavy metal oxide second major component selected fromthe group consisting of PbO, BiO₁.5, SbO₁.5, TlO₀.5, and combinationsthereof. Neither Ga₂ O₃ nor TeO₂ is mentioned in the patent and theworking examples containing Sb₂ O₃ are quite outside those of thepresent invention.

In "Third Order Nonlinear Integrated Optics," Journal of LightwaveTechnology, 6, pp. 953-967, June 1988, G. I. Stegeman, E. M. Wright, N.Finlayson, R. Zanoni, and C. T. Seaton discuss, in generic terms, thescientific principles that make possible the construction of nonlinearoptical devices from optical waveguides, namely Mach-Zehnderinterferometers, grating and prism couplers, grating reflectors,directional couplers, and mode sorters. Though many of these devicescoincide with those of the instant invention, no reference is made tospecific glass compositions that are suitable for such devices.

J. E. Stanworth in "Tellurite Glasses", Journal of the Society of GlassTechnology, 36, pages 217-241(1952), describes a survey of severaltellurite glass composition systems; viz., barium tellurite glasses,lead tellurite glasses, glasses containing TeO₂, SeO₂, and PbO, glassescontaining TeO₂, PbO, and BaO, glasses containing TeO₂, PbO, and Li₂ O,glasses containing TeO₂, PbO, and Na₂ O, glasses containing TeO₂, PbO,and Cb₂ O₅, glasses containing TeO₂, PbO, and P₂ O₅, glass containingTeO₂, V₂ O₅, and PbO or BaO, glasses containing TeO₂, BaO, and As₂ O₅,glasses containing TeO₂, PbO, and MoO₃, glasses containing TeO₂, PbO,and WO₃, glasses containing TeO₂, PbO, and ZnF₂, glasses containingTeO₂, PbO, and MgO, TiO₂, GeO₂, or La₂ O₃, glasses containing TeO₂, PbO,and CeO₂, or ZrO₂, glasses containing TeO₂ , PbO, MnO₂, or CuO, andglasses containing TeO₂, PbO, and B₂ O₃. There is no reference to any ofthe glasses exhibiting optical nonlinear behavior and Tl₂ O is nowherementioned.

British Pat. No. 736,073 (Stanworth) is essentially a foreshortenedreprise of the previous literature reference, the same compositionsystems being disclosed. Again, there is no reference to any of theglasses exhibiting optical nonlinearity and Tl₂ O is nowhere mentioned.

DESCRIPTION OF PREFERRED EMBODIMENTS

Table I records a group of glass compositions melted on a laboratoryscale and reported in terms of parts by weight on the oxide basisillustrating the present invention. Because the sum of the individualcomponents totals or very closely approximates 100, for all practicalpurposes the tabulated values may be deemed to represent weight percent.Table IA recites the same group of glass compositions expressed in termsof cation percent. The actual batch ingredients may comprise anymaterials, either oxides or other compounds, which, when meltedtogether, are converted into the desired oxides in the properproportion. For example, either TlNO₃ or Tl₂ CO₃ may be employed as thesource of thallium.

The batch ingredients were compounded, tumble mixed together to assistin securing a homogeneous melt, and then charged into gold, platinum, orVYCOR® brand crucibles. After placing lids thereon, the crucibles wereintroduced into a furnace operating at about 800° to 900° C. and thebatches were melted for about 15 to 60 minutes. The melts weresubsequently poured into steel or graphite molds to yield glass discshaving a diameter of about 1" and a thickness of about 1 cm which weretransferred immediately to an annealer operating at about 175° to 300°C.

Whereas the above description illustrates melting batches and formingglass bodies from those melts on a laboratory scale only, it must berecognized that large scale melts can be carried out in commercialmelting units and glass articles formed therefrom using conventionalglass forming equipment. Therefore, it is only necessary that the batchmaterials be melted at a temperature and for a time sufficient to obtaina homogeneous melt.

Table I also records the Verdet constant (V₆₃₃) at 633 nm expressed interms of min/Oe-cm, the electrooptic coefficient (B₆₃₃) at 633 nmexpressed in terms of cm/kV², the glass transition temperature (Tg)expressed in terms of ° C, the glass crystallization temperature (T_(x))expressed in terms of ° C., and the effective working range of the glass(T_(x) -T_(g)) expressed in terms of ° C. determined in accordance withmeasuring techniques conventional in the glass art. Also included is thethird order nonlinear susceptibility [X.sup.(3) ] expressed in terms of10⁻¹⁴ esu as estimated from the electrooptic coefficient (B₆₃₃).

                  TABLE I                                                         ______________________________________                                        1        2      3      4    5    6    7    8    9                             ______________________________________                                        Tl.sub.2 O                                                                          --     5.7    11.4 17.1 21.1 46.9 57.9 48.6 50.9                        PbO   73.2   65.4   59.7 54.0 51.8 --   --   --   --                          TeO.sub.2                                                                           21.8   24.1   24.1 24.1 22.1 52.8 47.8 24.3 41.1                        SiO.sub.2                                                                           4.9    4.8    4.8  4.8  5.0  0.3   0.3  0.3  0.4                        WO.sub.3                                                                            --     --     --   --   --   --   --   26.9 --                          ZnO   --     --     --   --   --   --   --   --    7.7                        V.sub.633                                                                           0.136  0.143   0.14                                                                               0.15                                                                               0.15                                                                               0.15                                      B.sub.633                                                                           1.3    0.8    1.1  1.4  --   2.0                                        T.sub.g                                                                             280    246    243  215  189  --                                         T.sub.x                                                                             365    383    353  317  333  --                                         ΔT                                                                             85    137    110  102  144  --                                         X.sup.(3)                                                                            22     14     19   25  --   36                                         ______________________________________                                    

                  TABLE IA                                                        ______________________________________                                        (Cation Percent)                                                              1        2      3      4    5    6    7    8    9                             ______________________________________                                        Tl.sub.2 O                                                                          --      5.0   10.0 15.0 18.0 39.5 44.5 45.6 40.5                        PbO   60.0   55.0   50.0 45.0 42.0 --   --   --   --                          TeO.sub.2                                                                           25.0   25.0   25.0 25.0 25.0 59.5 54.5 30.4 43.5                        SiO.sub.2                                                                           15.0   15.0   15.0 15.0 15.0  1.0  1.0  0.9  1.0                        WO.sub.3                                                                            --     --     --   --   --   --   --   23.1 --                          ZnO   --     --     --   --   --   --   --   --   16.0                        ______________________________________                                    

As can be seen from the above data, the inclusion of Tl₂ O intellurite-leased glass compositions imparts a number of advantageousproperties thereto, when compared with standard HMO glass as illustratedby Example 1. Thus, the following general observations can be made:

(a) the Verdet content of the glass increases as the concentration ofTl₂ O is increased;

(b) at levels of above about 15 cation % TlO₀.5, the third ordernonlinear susceptibility of the glass increases, with Examples 6 and 7demonstrating significantly higher values of X.sup.(3) ; and, veryimportantly,

(c) the presence of TlO₀.5 greatly broadens the working range of theglasses; commonly to values in excess of 100° c., thereby facilitatingthe formation of glass articles such as fibers.

Based upon an overall review of physical properties, coupled withimproved melting and forming characteristics, the preferred compositionsof the present inventive glasses consist essentially, expressed in termsof cation percent on the oxide basis, of 20-50% TeO₂, 2-20% SiO₂, 5-45%TlO₀.5, and 5-58% PbO, wherein PbO+TlO₀.5 ≧48%, TeO₂ +SiO₂ ≧30%, and thesum of TeO₂ +SiO₂ +TlO₀.5 +PbO constitutes at least 70% of the totalcomposition.

Our most preferred composition is Example 4.

Application of the aforementioned glasses to devices may necessitate thesynthesis of an optical waveguide structure, As is generally known toone skilled in the art, in order for such a structure to operateproperly there must be a difference in refractive index existing betweenthe core and cladding. This difference is dependent upon the type ofwaveguide, i.e., a single mode or multimode, and upon the use of thewaveguide, e.g., whether the fiber is used in a straight configurationor whether bent. Accordingly, whereas both the core and the claddingglasses can be prepared from compositions within the same inventiveranges, it will be appreciated that the compositions of the core andcladding glasses will be sufficiently dissimilar to achieve a desireddifference in refractive index between the core and cladding.

While the principles of the instant invention have been described abovein connection with specific embodiments and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example, and not as a limitation on the scope of theinvention. Said principles may be otherwise embodied within the scope ofthe following claims.

We claim:
 1. A glass exhibiting a high Verdet constant and high opticalnonlinearity, along with high transmission in the visible and infraredportions of the radiation spectrum, consisting essentially, expressed interms of cation percent on the oxide basis, of 5-74.5% TeO₂, 0.5-20%SiO₂, 4-50% TlO₀.5, and 0-71% PbO, wherein TlO₀.5+PbO≧ 25%, TeO₂ +SiO₂≧25%, and TeO₂ +SiO₂ +TlO₀.5 +PbO constitutes at least 65% of the totalcomposition.
 2. A glass according to claim 1 wherein said glass alsocontains up to 35 cation % total of at least one member of the group inthe indicated proportion consisting of up to 35% WO₃ and up to 25% ZnO.3. A glass according to claim 1 consisting essentially of 20-50% TeO₂,2-20% SiO₂, 5-45% TlO₀.5, and 5-58% PbO, wherein PbO+TlO₀.5 ≧48%, TeO₂+SiO₂ ≧30%, and the sum of TeO₂ +SiO₂ +TlO₀.5 +PbO constitutes at least70% of the total composition.